The long-range goal of this research is to develop an understanding of the mechanisms underlying the adverse health effects and toxicity resulting from exposure to complex mixtures of polycyclic aromatic hydrocarbons (PAH) and chromium, a carcinogenic metal often found as an environmental co-contaminant with PAHs. The objectives of this research project are, (1) to elucidate the mechanisms by which chromium affects inducible gene expression, and (2), to evaluate the effect of mixtures of benzo[a]pyrene (B[a]P), a prototypical PAH, and chromium on the post-translational modifications of chromatin remodeling proteins associated with epigenetic modulation of gene expression. Chromium exposure has been shown to alter inducible gene expression, to form chromium-DNA adducts and chromium-DNA cross-links, and to disrupt transcriptional activator/coactivator complexes. During the previous 4 years of this grant we have shown that chromium blocks gene expression by interfering with the assembly of productive transcriptional complexes at the promoters of inducible genes. We studied the effects of chromium on the expression of genes induced by B[a]P and showed that chromium disrupted the transcriptional regulation of phase I and phase II detoxification genes induced by B[a]P-dependent Ah receptor activation and the inducible expression of over 50 different genes involved in a variety of signal transduction pathways. These effects of chromium were the result of the inhibition of critical chromatin remodeling steps necessary for gene transactivation. Given the central role of histones in maintaining chromatin structure, for the next 5 years of this grant application we propose to test the hypothesis that exposure to mixtures of chromium and B[a]P causes specific histone modifications that generate transcriptionally inactive chromatin and induce non-additive gene expression effects that cannot be predicted from the individual effect of each mixture component. Results from this work will help to generate an understanding of the risks arising from exposure to chemical mixtures and to develop effective means to predict their health effects.